Murine erythroleukemia (MEL) cells are an established system for studying erythroid differentiation. While it is evident that inducers of differentiation must affect gene expression, their mechanism of action remains unknown. The investigators have recently determined that inducers increase the deadenylation and decrease the stability of Spi-1 mRNA, a transcription factor that is implicated in erythroleukemogenesis. Since, a general increase in mRNA deadenylation follows inducer exposure, this suggests that inducers may globally affect poly(A) metabolism. The poly(A)-binding protein (PABP) has been proposed to mediate the affects of mRNA polyadenylation. In MEL cells, the PABP appears to be present in limiting abundance and is found almost exclusively in polysomes in both control and inducer exposed cells. Since inducers decrease the amount of mRNA in polysomes and increase the amount of mRNA in subpolysomal fractions, these data suggest that inducers result in a net loss of PABP from polysomal mRNAs. The investigators propose that the post-transcriptional effect of inducers is mediated by increasing the rate of removal of the PABP from mRNA poly(A) tails. To test this hypothesis, the proposed experiments will focus on the translational effect of these agents and will: 1) Quantitate the total amount of poly(A) and PABP in fractionated cell extracts to determine if inducers cause a net decrease in the amount of PABP/poly(A) in the cells; 2) Determine the effects of inducers on expression of the PABP and identify the mechanisms that regulate its expression in inducer-exposed cells; 3) Determine the effect of forced overexpression of the PABP on translation, deadenylation and stability of mRNAs in MEL cells and determine if this affects the induction of cell differentiation; and 4) Determine if mRNAs require a 5'-terminal oligopyrimidine tract to be subject to the translational effect of inducers. These experiments may provide significant insight into the function of the PABP and the role that this protein plays in the changes in gene expression that follow inducer exposure of MEL cells